The easiest way to clean spring onions or leeks, which can get sandy mud stuck in their many layers, is to slice them in half lengthwise, then cut them crosswise into 1/4" semicircles. Place in a bowl and fill with warm water, swish them around a bit, and let them soak for a few minutes. Lift them out of the water. The sandy bits will have sunk to the bottom, and you will have yourself some squeaky clean alliums.

Melt the butter in a large soup pot or Dutch oven over medium heat. Add the alliums and sweat until soft. Add the turnips and potatoes and toss to coat in the butter, cooking and stirring for a minute or two. Add the thyme, bay leaves, salt, pepper, and stock. Bring to a boil, and then reduce to a simmer. Cook, partially covered, until the vegetables are fork tender, 15 or 20 minutes. Add the chopped turnip greens and simmer 10 minutes more. Serve with a splash of cream and a few fresh thyme leaves.